From: Mark Rutland <mark.rutland@arm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, will@kernel.org,
catalin.marinas@arm.com, Mark Brown <broonie@kernel.org>,
James Clark <james.clark@arm.com>, Rob Herring <robh@kernel.org>,
Marc Zyngier <maz@kernel.org>,
Suzuki Poulose <suzuki.poulose@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
linux-perf-users@vger.kernel.org
Subject: Re: [PATCH V11 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE
Date: Fri, 9 Jun 2023 13:37:32 +0100 [thread overview]
Message-ID: <ZIMdDMSjnNfx6q+n@FVFF77S0Q05N.cambridge.arm.com> (raw)
In-Reply-To: <006be03c-43da-6482-6bc4-83fe65dcd706@arm.com>
On Fri, Jun 09, 2023 at 10:00:09AM +0530, Anshuman Khandual wrote:
> >> +static u64 branch_type_to_brbcr(int branch_type)
> >> +{
> >> + u64 brbcr = BRBCR_EL1_DEFAULT_TS;
> >> +
> >> + /*
> >> + * BRBE need not be paused on PMU interrupt while tracing only
> >> + * the user space, bcause it will automatically be inside the
> >> + * prohibited region. But even after PMU overflow occurs, the
> >> + * interrupt could still take much more cycles, before it can
> >> + * be taken and by that time BRBE will have been overwritten.
> >> + * Let's enable pause on PMU interrupt mechanism even for user
> >> + * only traces.
> >> + */
> >> + brbcr |= BRBCR_EL1_FZP;
> > I think this is trying to say that we *should* use FZP when sampling the
> > kernel (due to IRQ latency), and *can* safely use it when sampling userspace,
> > so it would be good to explain it that way around.
>
> Agreed, following updated comment explains why we should enable FZP
> when sampling kernel, otherwise BRBE will capture unwanted records.
> It also explains why we should enable FZP even when sampling user
> space due to IRQ latency.
>
> /*
> * BRBE should be paused on PMU interrupt while tracing kernel
> * space to stop capturing further branch records. Otherwise
> * interrupt handler branch records might get into the samples
> * which is not desired.
> *
> * BRBE need not be paused on PMU interrupt while tracing only
> * the user space, because it will automatically be inside the
> * prohibited region. But even after PMU overflow occurs, the
> * interrupt could still take much more cycles, before it can
> * be taken and by that time BRBE will have been overwritten.
> * Hence enable pause on PMU interrupt mechanism even for user
> * only traces as well.
> */
> brbcr |= BRBCR_EL1_FZP;
Thanks; I think that's a lot clearer!
Mark.
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2023-06-09 12:38 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-31 4:04 [PATCH V11 00/10] arm64/perf: Enable branch stack sampling Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 01/10] drivers: perf: arm_pmu: Add new sched_task() callback Anshuman Khandual
2023-06-05 7:26 ` Mark Rutland
2023-05-31 4:04 ` [PATCH V11 02/10] arm64/perf: Add BRBE registers and fields Anshuman Khandual
2023-06-05 7:55 ` Mark Rutland
2023-06-06 4:27 ` Anshuman Khandual
2023-06-13 16:27 ` Mark Rutland
2023-06-14 2:59 ` Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 03/10] arm64/perf: Add branch stack support in struct arm_pmu Anshuman Khandual
2023-06-05 7:58 ` Mark Rutland
2023-06-06 4:47 ` Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 04/10] arm64/perf: Add branch stack support in struct pmu_hw_events Anshuman Khandual
2023-06-05 8:00 ` Mark Rutland
2023-05-31 4:04 ` [PATCH V11 05/10] arm64/perf: Add branch stack support in ARMV8 PMU Anshuman Khandual
2023-06-02 2:33 ` Namhyung Kim
2023-06-05 2:43 ` Anshuman Khandual
2023-06-05 12:05 ` Mark Rutland
2023-06-06 10:34 ` Anshuman Khandual
2023-06-06 10:41 ` Mark Rutland
2023-06-08 10:13 ` Suzuki K Poulose
2023-06-09 4:00 ` Anshuman Khandual
2023-06-09 9:54 ` Suzuki K Poulose
2023-06-09 7:14 ` Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE Anshuman Khandual
2023-06-02 1:45 ` Namhyung Kim
2023-06-05 3:00 ` Anshuman Khandual
2023-06-05 13:43 ` Mark Rutland
2023-06-09 4:30 ` Anshuman Khandual
2023-06-09 12:37 ` Mark Rutland [this message]
2023-06-09 4:47 ` Anshuman Khandual
2023-06-09 12:42 ` Mark Rutland
2023-06-09 5:22 ` Anshuman Khandual
2023-06-09 12:47 ` Mark Rutland
2023-06-09 13:15 ` Mark Rutland
2023-06-09 13:34 ` James Clark
2023-05-31 4:04 ` [PATCH V11 07/10] arm64/perf: Add PERF_ATTACH_TASK_DATA to events with has_branch_stack() Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 08/10] arm64/perf: Add struct brbe_regset helper functions Anshuman Khandual
2023-06-02 2:40 ` Namhyung Kim
2023-06-05 3:14 ` Anshuman Khandual
2023-06-05 23:49 ` Namhyung Kim
2023-06-13 17:17 ` Mark Rutland
2023-06-14 5:14 ` Anshuman Khandual
2023-06-14 10:59 ` Mark Rutland
2023-05-31 4:04 ` [PATCH V11 09/10] arm64/perf: Implement branch records save on task sched out Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 10/10] arm64/perf: Implement branch records save on PMU IRQ Anshuman Khandual
2023-06-09 11:13 ` [PATCH V11 00/10] arm64/perf: Enable branch stack sampling Anshuman Khandual
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ZIMdDMSjnNfx6q+n@FVFF77S0Q05N.cambridge.arm.com \
--to=mark.rutland@arm.com \
--cc=acme@kernel.org \
--cc=anshuman.khandual@arm.com \
--cc=broonie@kernel.org \
--cc=catalin.marinas@arm.com \
--cc=james.clark@arm.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-perf-users@vger.kernel.org \
--cc=maz@kernel.org \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=robh@kernel.org \
--cc=suzuki.poulose@arm.com \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).